Fuel Filler Cap: Description and Operation

Fuel Tank Pressure/Vacuum Filler Cap:

The pressure-vacuum filler cap is designed to hold a slight pressure on the fuel. The evaporation rate decreases under higher pressures. When vacuum in the tank exceeds a pressure differential of 0.5 psi to atmospheric pressure, the vacuum relief valve will allow atmospheric pressure to enter the tank. If fuel tank pressure becomes 1.8 psi above atmospheric pressure, the pressure relief portion of the cap will open to the atmosphere. A rubber seal around the inside lip of the cap prevents fuel vapor from escaping to the atmosphere, as well as prevent fuel from spilling in the event of vehicle roll-over.